1. Field of the Invention
The present invention generally relates to a non-invasive, in-situ method and apparatus for monitoring and controlling etch processes. In particular, the invention includes controlling etch uniformity and relative etch rate, as well as detecting etch endpoint and monitoring gas-phase species above a substrate to identify particulate contamination.
2. Description of the Prior Art
Many semiconductor manufacturing processes are sensitive to changes in wafer temperature. The ability to determine the substrate temperature and to monitor changes in its value over long periods of time is important in process characterization and control. In plasma processing, energetic ion bombardment and radio frequency (RF) induced eddy currents lead to an increase in wafer temperature. In addition, wafer temperature can increase from reaction exothermicity. For example, Schwartz et al. in "Plasma Processing," ed. R. G. Freiser and C. J. Mogab, pp. 133-154, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ (1981), reported that an exothermic etching reaction occurs when etching silicon in a chlorine ambient, and Donner et al. in J. EIectrochem. Soc., 133:151-155 (1986), reported that an exothermic etching reaction occurs when etching aluminum in a chlorine ambient.
Several techniques have been used for monitoring plasma etching conditions while etching a substrate. For example, optical emission spectroscopy (OES) and laser induced fluorescence (LIF) techniques have been used to detect the etch endpoint in semiconductor manufacturing. However, OES and LIF techniques both use gas-phase species as an indirect indicator of the reaction occurring on the substrate surface and do not directly monitor conditions on the substrate. Laser interferometry has been widely used as a technique to monitor the progress of an etching reaction; however, this technique relies on the presence of a smooth, reflective "test point" (fiducial) on the wafer. Hoekstra, in IBM Tech. Discl. Bull., 11/73, p. 1721 (1973) descrives a process of etch reaction control which utilizes localized heating to maintain uniformity of the etching process across a wafer surface. Hoekstra monitors the etch rate at various points across the wafer electrically and controls the rate of etching through external application of heat.
Infrared thermography is a well known technique for non-invasive (non-contact) monitoring of the temperature of materials in harsh environments. In infrared thermography, a measurement of the intensity of infrared radiation (IR) emanating from an object and a knowledge of the emissivity of the material from which the object is made are used to determine the temperature of the object. IR thermography has made been used in many manufacturing processes. For example, U.S. Pat. No. 3,718,757 discloses the use of an IR television camera to monitor a crystal pulling operation wherein the detected video signal is indicative of the temperature at a surface point being analyzed. Similarly, Kaplan in Photonics Spectra, Dece. 1987, pp. 92-95, discusses the use of IR sensors and scanners in metal extrusion processes. IR thermography has also been used in the microelectronics field. For example, Hughes Aircraft Company manufactures a thermal imaging microscope and video system called the Probeye.RTM. which may be used to detect excessive junction temperatures.
U.S. Pat. No. 3,664,942 to Havas et al., which issued to IBM Corporation in 1972, discloses detecting the etch endpoint in a sputter etching procedure with an IR camera. In Havas et al., the emissivity of an object is monitored as layers of material are removed. Emissivity is a physical property of a material (like thermal conductivity) and reflects the ratio of radiation intensity from a surface to the radiation intensity at the same wavelength from a black body at the same temperature. An important drawback of a monitoring system which only monitors changes in emissivity (like Havas et al.) is that it cannot be used to distinguish between two layers of a stack which have similar emissivity levels at the sensing wavelength (e.g., SiO.sub.2, and Si in a stack at sensing wavelengths of 2-4 .mu.m). Moreover, the actual substrate temperature may remain unchanged while the emissivity may change greatly due to different materials (with different emissivity) being etched; therefore, substrate temperature conditions are not monitored.
U.S. Pat. No. 4,936,967 to Ikuhara et al. shows a system that analyzes a wavelength of the plasma spectrum above a substrate during etching rather than monitoring the substrate. While Ikuhara contemplates monitoring the plasma, it does not provide any means for identifying suspended particles in the plasma which may compromise the parts being etched.